1. Field of the Invention
This invention relates to a process and apparatus for metallurgically treating high performance steel gears by thermomechanical means to produce high strength and accurate contact surfaces using controlled deformation net shape finishing techniques.
2. Discussion of the Prior Art
Highly loaded precision gears are normally manufactured by carburizing the surface layers of low carbon low alloyed steel gears, and reaustenitizing the entire gear and hardening by rapid quenching to below the temperature at which diffusionless transformations occur that result in the hardened martensitic structures. The hardened gears are then finished to net shape by hard finishing operations. A method was proposed in U.S. Pat. No. 4,373,973 in which a carburized gear is reaustenitized and quenched to above the M.sub.s temperature, roll finished, and then quenched to martensite prior to diffusional decomposition of the metastable austenite. However, no specific process details or apparatus are described in that patent which can accomplish this process.
In reducing the concept of U.S. Pat. No. 4,373,973 to practice, several inventions were necessary in both process control and apparatus to produce the metallurgical and dimensional accuracy requirements of precision gears. These inventions have been disclosed in a separate invention disclosure, commonly assigned application Ser. No. 07/829,187, filed Jan. 31, 1992, of M. Amateau et al., entitled "Apparatus and Method For Precision Gear Finishing by Controlled Deformation", now U.S. Pat. No. 5,221,513 issued Jun. 22, 1993 the entire disclosure thereof being incorporated herein by reference. However, for ultra-high precision gears, an even closer control of the deformation process is required of the material flow pattern, degree and depth of deformation, and the metallurgical conditions of the gear tooth surface and subsurface layers. For instance, the gear finishing process as described in the disclosure of U.S. Pat. No. 5,221,513 utilizes in-feed and through-feed motions of the workpiece in relation to a single gear rolling die. The deformation mechanism related to such a rolling process with a single rolling die results in different material flow patterns on either side of the workpiece teeth, which can adversely effect the behavior of high performance gears. Further, gear roll finishing using a single rolling die can result in excessive deflections in the workpiece support spindle, which must be compensated for by prior machine settings.
By use of two rolling dies positioned on diametrically opposing sides of the workpiece, the material flow patterns as well as the high in-feed rolling forces can be balanced, resulting in a better control of the deformation process. Our invention is different from the conventional gear roll finishing equipment using two rolling dies, in that, for the latter, the first rolling die is typically held with a fixed axis and the second rolling die is moved, thereby applying the in-feed force and rolling action on the workpiece, and moving the workpiece towards the fixed rolling die at preset speeds. The required amount of deformation is controlled by setting a dead stop at a predetermined location, where the in-feed motion ends. Such a gear finishing process using two rolling dies, one fixed and the other moving for the in-feed motion, is generally used for cold rolling of uncarburized steels only, and is further limited to helical gears only.
To achieve the ausform-strengthening of surface layers of carburized parallel axis gear teeth for high performance applications, both in-feed and through-feed motions are required between the workpiece and the two rolling dies in a coordinated and controlled manner, and such a controlled deformation must be achieved with surface layers of the workpiece maintained in the metastable austenitic condition. The large in-feed and through-feed forces necessary to roll finish spur and helical gears to the high dimensional accuracy require a rigid through-feed mechanism holding the workpiece on a fixed axis, and coordinated and controlled in-feed motion of the two rolling dies towards the fixed axis workpiece. The degree of deformation must be controlled to very close tolerances by precise monitoring and control of the movements of each of the two rolling dies with respect to the workpiece. Further, the workpiece axis as well as the axes of the two rolling dies must be precisely aligned to achieve the high lead and profile accuracy specified for ultra-high precision gears. In addition, as the thermomechanical processing of the workpiece must be performed in a thermally stable bath to maintain the workpiece gear surfaces in the desired metastable austenitic condition during the forming process, any adjustments to the alignments between the workpiece and the rolling die axes must be made with the rolling apparatus maintained at the forming temperature. Moreover, the degree of deformation and metallurgical structures of the gear surface layers must all be maintained in a precisely controlled manner. The surface reaustenitization, the transformation to metastable austenitic condition, and the subsequent transformation to martensite, must be performed in a timely and controlled manner to achieve the optimum metallurgical condition at each stage of the thermomechanical processing.